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Abstract

We describe an ultrabroadband IR-visible sum-frequency (SF) setup that allows simultaneous acquisition of the entire vibrational spectrum of water molecules at mineral surfaces in the OH stretching region without ever tuning the IR laser pulses. Our newly developed 800-nm pumped noncollinear optical parametric amplifier (NOPA) generates broadband mid-IR pulses (~1800-3500 nm, or ~2900 – 6000 cm−1) with bandwidths >600 cm−1 at half-maximum at near 3500 cm−1. Using the ultra-broadband IR NOPA, we constructed a sum-frequency vibrational spectrometer that allowed the acquisition of spectra of the OH stretches of water at hydrophilic and hydrophobic silica surfaces, over the frequency range ~2900 – 3800 cm−1, within 60 s, much shorter than with scanning SFG spectrometers. The ultra-broadband SFG spectrometer reported here can be potentially applied to time-resolved measurements of kinetics at interfaces.

Acquisition of sum-frequency spectra from silica/water interfaces by scanning the VIS-IR delay Δτ shown on the example of water at hydrophobic octadecyltrichlorosilane-coated fused silica (OTS-IRFS) surface [15]. (a) ppp SF-spectra from Au-IRFS prism at selected VIS-IR delays (spectra are shown spaced at 267 fs); (b) ppp-SF-spectra from H2O/OTS-IRFS prism at the same selected values of Δτ as for the Au-IRFS reference prism in a).

(a): Normalized SFG-spectrum of the neat fused silica/neat water (pH~6) interface (blue solid line) in ppp-polarization, shown together with the reference spectrum of the ultra-broadband IR pulses (dotted grey line). The green solid line is the SFG spectrum from the neat fused silica/D2O interface shown on the same scale with the H2O/fused-silica spectrum. (b): Normalized SFG-spectrum of ODS-coated silica / neat water (pH~6) interface (blue solid line) in ppp-polarization, together with the reference spectrum of the ultra-broadband IR pulses (dotted grey line).

Left: raw spectra obtained at a single IR-VIS delay from the H2O/OTS-IRFS interface (“sample”, red) and the gold-coated IRFS (“ref”, black). The sample spectrum was acquired in 60 sec. Right: “single-acquisition” SFG spectrum of H2O/OTS-IRFS interface (blue) obtained by normalizing the “sample” spectrum in the left-hand side graph by the “ref” spectrum. Red dotted line in the background – SFG spectrum from water/OTS interface obtained by integrating SF-signal as the delay between IR and visible pulses was scanned. For convenience, the single-acquisition spectrum is truncated at ~3000 cm−1 due to the large noise in the C-H stretch region (grey dotted line).

Left: similar to Fig. 6, raw spectra obtained at a single IR-VIS delay from the H2O/bare IRFS interface (“sample”, red) and the gold-coated IRFS (“ref”, black). The sample spectrum was acquired in 60 sec. Right: “single-acquisition” SFG spectrum of H2O/bare IRFS interface (blue) obtained by normalizing the “sample” spectrum in the left-hand side graph by the “ref” spectrum. Red dotted line in the background – SFG spectrum from water/bare silica interface obtained by integrating SF-signal as the delay between IR and visible pulses was scanned.